WiMAX, short for worldwide interoperability for microwave access, is currently defined by the Institute of Electrical and Electronics Engineers, or IEEE, 802.16-series specification. Mobile WiMAX, under 802.16e, permits broadband wireless access for mobile users. Under Mobile WiMAX, orthogonal frequency division multiple access (OFDMA) is used to multiplex different users with time-frequency resource allocation in the same transmission frame. Each user is allocated one or more sub-channels made up of a fixed number of narrowband sub-carriers. Because the sub-carriers are orthogonal to one another, interference levels are generally low between them in a single cell/sector.
Where the same sub-carriers are in the same time slots within nearby cells/sectors, collisions (interference) may occur. Under the 802.16e standard, the same frequency band may be used in other, usually non-adjacent cells/sectors, known as fractional frequency reuse (FFR). Under FFR, cell edge users are distinguished from cell center users, with cell edge users operating with a fraction of the available sub-channels (known as frequency reuse 3 or R3) while the cell center users operate with all available sub-channels (known as frequency reuse 1 or R1).
It had been shown that frequency reuse 1 typically provides much better spectrum efficiency than frequency reuse 3. Therefore, future orthogonal frequency division multiplexing (OFDM) based cellular systems (such as WiMAX and LTE, short for long-term evolution) are evolving toward frequency reuse 1 in order to maximize the spectral efficiency. However, in systems deployed with frequency reuse 1, cell edge users will suffer severe performance degradation due to very strong co-channel interference (CCI) from neighboring sectors/cells, which leads to poor cell coverage.
Fractional frequency reuse enables multiple frequency reuse factors in one system, and allows users to choose suitable reuse value according to their channel quality. Fractional frequency reuse thus helps the system yield high spectrum efficiency while maintaining decent coverage.